The cost of precious metals, such as gold, makes recovery or recycling of these materials economically viable and desirable. Gold is typically recovered from ores and other impure sources using cyanide, aqua regia or smelting. Such methods suffer from inter alia, toxicity issues, disposal costs and high energy input.
Prior art patents addressing the problem of improving efficiency in gold extraction and recovery are numerous. For example, U.S. Pat. No. 3,834,896 describes a process for recovering gold involving injecting chlorine into an aqueous slurry of carbonaceous ore at high temperature in the presence of iron, aluminium or gallium promoters.
U.S. Pat. No. 4,723,998 discloses a two-step process for extracting gold from carbonaceous or metal oxide based ores. The process comprises using chlorine to dissolve the gold from the ores and subsequently absorbing the gold on to ion exchange resins.
U.S. Pat. No. 3,495,976 communicates a method of recovering gold that has been plated or coated on to non-ferrous metals such as tungsten, molybdenum, or copper. The gold plated material is treated with an aqueous solution of potassium iodide and dissolved iodine. The gold is recovered by adding conc. sulphuric acid and distilling of the iodine. When all the iodine has been removed, the gold is separated from the remaining solution as a powder.
U.S. Pat. No. 3,957,505 discloses a process for extracting gold from gold bearing material comprising: treating the gold bearing material in an aqueous solution consisting essentially of iodine and a water soluble iodide salt to dissolve gold from said gold bearing material; mixing a reducing agent with said aqueous solution to reduce dissolved gold iodide salts to gold metal and precipitate said gold metal in substantially pure form from said aqueous solution. The precipitated gold metal is removed from the aqueous solution. An oxidizing agent is subsequently added to the aqueous solution to restore the solution to substantially its original condition for dissolving gold from further gold bearing material.
G.B. Patent No. 20471 discloses a method of extracting gold from ores thereof. The method discloses utilising an undivided electrolytic cell to generate a leaching material. Once the gold has been leached from the ore it is subsequently electrodeposited on the cathode. This method suffers in that the cathode has to be removed from the electrochemical cell to recover the gold and the cell will have to be cleaned out regularly to remove unwanted sludge, salts and other contaminants.
Hoffmann, J O M, Springer New York, vol 44, no. 7 p 43-48 describes methods for recovering precious metals from electronic scrap involving slurrying the scrap in water and sparging chlorine gas into the slurry. WO 01/83835 A2 describes a gold recovery process in which gold scrap is mixed with water and hydrocloric acid and chlorine gas is blown into the reactor, to dissolve the gold. Both methods use large amounts of water.
In gold leaching, using for example cyanide, it is essential to carry out the reaction in an aqueous system to facilitate ionisation of the sodium cyanide used to cyanide ions. Waste electronic scrap contains irregularly shaped pieces and will take up more volume than it would if compacted. But if compacted the leachant solution could not act on all the surfaces.
As an example, 382 grams of computer connector slots occupied a volume of 1000 cm3 in a beaker. To fill the same beaker to the 1000 cm3 mark required an additional volume of 770 cm3 of water. Based on these figures, 1 tonne of waste electronic scrap would occupy a volume of approximately 2.6 m3 and would require a volume of 2 m3 leachant to fill the container. In practical terms a larger tank with a larger volume of leachant would be needed to allow for agitation.
In contrast, gaseous chlorine as used in the present invention can circulate freely and penetrate into small nooks and crannies of the electronic scrap to leach and dissolve the surface gold. Much smaller volumes of water can be used to merely moisten the surfaces to facilitate reaction.
Notwithstanding the state of the art there remains a need for alternative methods for recovering gold that mitigate some or all of the above mentioned problems.